(7 - 9*b)*(7 + 9*b)
1*(b + 7/9)*(b - 7/9)
(1*(b + 7/9))*(b - 7/9)
2 49 - 81*b
49 - 81*b^2
(7.0 + 9.0*b)*(7.0 - 9.0*b)
-(-7 + 9*b)*(7 + 9*b)